1. Field of the Invention
The present invention relates to a method for separating phenols and bases from coal tar oils.
2. Brief Description of the Background of the Invention Including Prior Art
Carbolic acid oil boiling between 180.degree. and 210.degree. C. and obtained in an amount of from about 2 to 3% of the hard coal tar during the primary distillation comprises in addition to homologues of benzene, naphthalene and bases up to 35% phenols, which comprise a mixture of phenol, creosols and xylenols (Franck, Collin: Bituminous Coal Tar, p. 74) In addition the filtered naphthalene oil, with a phenol content up to 25%, obtained in particular during the crystallization of naphthalene and the light oil containing up to 10% phenols are used to obtain phenols. The most important method for obtaining phenol from these coal tar oils is sodium hydroxide extraction (Franck, Collin: Bituminous Coal Tar, pp. 75-77). The method is based on the fact that phenols are weak acids and therefore they form water soluble salts with bases and that the phenols can be precipitated again with stronger acids such as for example, carbonic acid.
The phenol containing coal tar oil is relieved of the phenol with an 8 to 12% aqueous sodium hydroxide solution in two stages. The phenols pass during this process into the sodium hydroxide solution under formation of phenolates. The method is performed in most cases at about 50.degree. C. in order to achieve a quicker separation of sodium hydroxide solution and oil.
It is not advisable to employ a lye of higher concentration because in addition neutral oils are dissolved by solubilization into the aqueous phase with increasing content in phenol in the phenolate lye.
The saturated or, respectively, nearly completely saturated phenolate lye is separated from adhering neutral oils by steam distillation until clearing. Following, the phenols are precipitated from the phenolate lye cleared by steam distillation by introduction of carbon dioxide. The carbonic acid required for this purpose is generated in a lime kiln, which is a construction part of the production unit.
After the precipitation, the raw phenol separates above the aqueous sodium solution and can be separated from the sodium solution. The sodium solution is made caustic with lime, which is generated by firing of limestone, and the calcium carbonate generated during caustification is filtered off from the recovered sodium lye. The lye is again set to an alkali content of from 8 to 12% and the lye is again returned into the cycle.
This process requires very much energy to carry out the evaporation and to operate a lime kiln and also involves a series of disadvantages:
(1) After the extraction of the carbolic acid oil with sodium hydroxide NaOH, there remains a residual phenol content of 0.5% in the oil, which cannot economically be recovered.
(2) The about 0.5% organic but non-phenolic parts are removed from the phenolate lye by a following steam distillation process until clearing. This is necessary in order to obtain pure phenol products required by the severe commercial delivery specifications. The heat requirements are reduced in fact by the use of exchange columns filled with packing filling material and by multiple stage evaporation systems, but nevertheless this process still entails substantial cost.
(3) In addition, a mixture of neutral oil tar bases and water is generated as a distillate. Also a small part of the phenol dissolved in the lye is driven off by the water vapor distillation. This part is on the one hand lost in the production process and in addition substantially contaminates the waste water. Also part of the bases are lost even though the distillate water is extracted with benzene or a similar solvent.
(4) The raw phenol, after the separation from the sodium lye, still contains sodium solution, which is removed substantially by renewed washing with water. Finally, the raw phenol still contains about 2% alkali at about 10 to 20% water. The water has to be removed before the fractioning of the raw phenol. The distillate water obtained thereby however is not added to the waste water, but is returned into the circulation system because of a very high phenol content.
(5) The lime sludge generated during the caustification of the sodium lye is very fine-grained and still contains about 50% water when in a form that is proof against sword cuts. (Translator's note: A knife (sword) would remain in place when stuck into the sludge.) Despite multiple washing with water on a vacuum filter, the residual content of 0.5% alkali remains.
(6) The recuperation of the alkali amount at maximum recuperation is only 95%.
The total requirement of chemicals for 1 ton of raw phenol comprises amounts on the average of:
625 kg limestone PA1 68 kg coke PA1 65 kg sodium hydroxide
It has been attempted to lower the loss in chemicals or the loss in phenols due to the precipitated lime or, respectively, to reemploy the precipitated lime in concrete factories or the like in order to avoid environmental pollution ensuing from rain washouts of slag heaps of the precipitated lime.
There has been no lack of attempts to change the method and to at least eliminate the operation of the lime kiln, for example, the Metasolvane method operated with aqueous methanol. However, this Metasolvane method has not been able to achieve widespread acceptance. The Phenoraffin method (Franck, Collin: Bituminous Coal Tar, pp. 77-79) employs sodium phenolate solution for extraction. The oversaturated phenolate lye is washed with benzene or toluene to remove bases and neutral oils. Raw phenol is obtained from the oversaturated phenolate lye by extraction with di-isopropyl ether. This method also has not succeeded in achieving permanent widespread acceptance.
The light oil boiling between 70.degree. and 200.degree. C. and generated in an amount of from 0.5 to 3% of the raw tar during the primary distillation contains 2 to 7% bases (Franck, Collin: Bituminous Coal Tar, p. 84). In addition, carbolic acid oil is employed for obtaining bases.
The bases contained in the oils are obtained by extraction with 25 to 35% sulphuric acid. The coal dissolved neutral oils are removed by washing with a solvent. Thereupon the bases are precipitated by neutralization of the base-sulphuric acid compound. For this purpose, mostly ammonia is employed. The bases are withdrawn and the aqueous solution is worked up by evaporation for obtaining ammonium sulphate (Franck, Collin: Bituminous Coal Tar, ppl 84-85). This procedural method also requires much energy and a large amount of chemicals.
In summary, it is noted that obtaining phenols and bases from tar oils is associated with a high expenditure of thermal energy and chemicals.